A standard door latch mounted on an outer door edge has a latching mechanism comprised mainly of a fork that can in a locked position engage around a jamb-mounted bolt and hold the door closed, and that can in an unlocked position release this bolt. The latching mechanism also normally has a pawl that is used to block the fork in the locked position, the fork being spring-loaded to move into the unlocked position when released by this pawl.
This pawl in turn can be moved by a release mechanism connected to an inside door handle or an outside door handle. Normally the outside handle acts against a lever on the latch and the inside handle is connected to its mechanism by a rod. The release mechanism itself is typically a lever system.
The standard door latch also is provided with a lock mechanism preventing the handles from acting on the pawl and allowing it to release the fork. This lock mechanism therefore comprises elements connected to the release mechanism and decoupling the handles from the pawl, so that when locked the handles can be operated but will do nothing.
A further feature now commonly seen is a device that allows all of the lock mechanisms to be operated centrally, typically by operation of the driver's door lock. Such an arrangement is frequently a retrofit, and merely is the provision of a servomotor in the door connected to the inside-lock control element. Thus this servomotor just duplicates the action of a person operating the inside-lock element.
These latches are fairly complicated and, as a result, expensive to manufacture and service. In the event of any failure of the latch it must normally be replaced in its entirety.
A disadvantage of the central-system latches is that a skilled thief can often open the door by reaching inside it to act on the linkage between the inside lock element and the release mechanism. The mechanism around the servomotor is particularly susceptible to such tampering. Such a lock system therefore frequently represents a loss in security.